The present invention relates to a method for recording/reproducing optical information recording medium wherein a light flux emitted from a light source is converged on an information recording surface by a light-converging optical system, and information is recorded on an optical information recording medium to be reproduced, or information on the information recording surface is reproduced, an optical pickup apparatus, an objective lens used for the foregoing, and a design method of the objective lens.
Recently, in association with practical use of a short wavelength red semiconductor laser, there has been advanced development of DVD (digital video disk, or digital versatile disk) representing an optical information recording medium which is the same in size as CD (compact disk) that is a conventional optical information recording medium (optical disk) and has higher capacity and higher density. In this DVD, numerical aperture NA of the objective lens on the optical disk side is 0.6 when the short wavelength semiconductor laser of 635 nm is used. Incidentally, in the DVD, a track pitch is 0.74 .mu.m and the shortest pit length is 0.4 .mu.m, which is less than a half of track pitch of 1.6 .mu.m and shortest pit length of 0.83 .mu.m of CD, representing that the DVD is of higher density. In addition to the CD and DVD mentioned above, optical disks of various standards, such as, for example, CD-R (recordable compact disk), LD (laser disk), MD (mini-disk), and MO (magneto-optical disk) have also been commercialized and have been spread. Table 1 shows a transparent substrate thickness and its necessary numerical aperture for each of various optical disks.
TABLE 1 ______________________________________ Transparent Necessary numerical substrate aperture NA (light Optical disk thickness (mm) source wavelength nm) ______________________________________ CD, CD-R (only for 1.20 0.45( = 780) reproducing) CD-R (recording and 1.20 0.50( = 780) reproducing) LD 1.25 0.50( = 780) MD 1.20 0.45( = 780) MO(ISO 3.5 inch 230 NB) 1.20 0.55( = 780) MO(ISO 3.5 inch 640 MB) 1.20 0.55( = 680) DVD 0.60 0.60( = 635) ______________________________________
Incidentally, for the CD-R, light source wavelength .lambda. is required to be 780 (.mu.m), but for the other optical disks, it is possible to use light sources having wavelengths other than those shown in Table 1, and in this case, necessary numerical aperture NA can be found in accordance with wavelength .lambda. of the light source to be used. For example, in the case of CD, necessary numerical aperture NA is approximated to .lambda. (.mu.m)/1.73 and in the case of DVD, necessary numerical aperture NA is approximated to .lambda. (.mu.m)/1.06.
Now, it is an age where various optical disks having different sizes, substrate thickness, recording densities, and wavelengths to be used exist in the market as stated above, and optical pickup apparatuses capable of handling various optical disks have been proposed.
As one of them, there has been proposed an optical pickup apparatus wherein a light-converging optical system capable of working with each of different optical disks is provided and the light-converging optical system is switched depending on an optical disk to be reproduced. However, in this optical pickup apparatus, plural light-converging optical systems are needed, resulting in a cost increase, and a driving mechanism for switching the light-converging optical system is needed, resulting in a complicated apparatus, and its switching accuracy is required, which is not preferable.
Therefore, there have been proposed various optical pickup apparatuses each employing a single light-converging optical system and reproducing a plurality of optical disks.
As one of them, TOKKAIHEI 7-302437 discloses an optical pickup apparatus wherein a refraction surface of an objective lens is divided into plural ring-shaped areas, and each divided area forms an image on one of optical disks having different thickness for reproducing.
In addition, TOKKAIHEI 7-57271 discloses an optical pickup apparatus wherein an objective lens designed to make wavefront aberration owned by a converged beam to be 0.07.lambda. or less is used in the case of a first optical disk with a transparent substrate having a thickness of t1, and the objective lens is defocused slightly in the case of a second optical disk with a transparent substrate having a thickness of t2, both for forming a light-converged spot.
However, in the optical pickup apparatus disclosed in TOKKAIHEI 7-302437, an incident light amount is divided to two focal points simultaneously by a single objective lens. It is therefore necessary to make the laser output to be high, which results in a cost increase. In the optical pickup apparatus disclosed in TOKKAIHEI 7-57271, on the other hand, there is caused an increase of the side lobe jitters when reproducing the second optical disk. In this case, in particular, the second optical disk is reproduced forcibly by the objective lens designed to make wavefront aberration to be 0.07.lambda. or less in the case of the first optical disk. Therefore, the numerical aperture which makes it possible to reproduce the second optical disk is naturally limited.